Is gravity keeps the planets in orbit around the sun
a. true
b. false

True. It's a bit more complex than simply that, but for the question you asked the answer is A.
Inertia also works in tandem with gravity, and plants orbit they don't revolve (kudos on that! Small pet peeve of mine). Newtons laws of physics will help you understand this more; remember to keep in mind we are talking about space so the suns mass has gravitational pull, but space has no gravity- So inertia plays an important part as well.